Microwave radiation is generally defined as that electromagnetic radiation having wavelengths between radio waves and infrared radiation. There is no sharp division but for this application we will use the phrase microwave radiation to define that band of electromagnetic radiation having wavelengths between 3 .mu.m and 30 cm corresponding to frequencies of about 1 GHz to about 1,000 GHz.
Microwave radiation can be forced to travel in specially designed waveguides. Microwave radiation can be transmitted through space or through the atmosphere in a microwave beam from a microwave antenna and the microwave energy can be collected with a microwave antenna.
Objects in the environment emit and reflect microwave radiation at various wavelengths within the microwave spectrum. The intensity of emission and reflection from an object depend on the object's temperature, composition and various other factors. As a consequence, images of objects can be obtained by utilizing a microwave antenna capable of measuring microwave intensity as a function of direction. Such devices are sometimes referred to as microwave cameras. Some of the Applicants were co-inventors of microwave cameras described in U.S. Pat. No. 5,365,237 issued on Nov. 15, 1994 and U.S. Pat. No. 5,121,124 issued on Jun. 9, 1992, both of which patents are incorporated herein by reference.
Many antennas currently exist for transmitting and receiving microwave radiation. Microwave antenna used for communication are very common. These devices typically comprise an open ended waveguide and a parabolic reflector or horn and they typically transmit a predetermined frequency in a predetermined direction. A good description of prior art antennas is found in Antenna Engineering Handbook, edited by Johnson and Janik and published by McGraw-Hill Book Company with offices in New York, N.Y.
It is well known that the beam direction of some types of antenna can be altered or "scanned" in one dimension by adjusting the frequency (or wavelength) of the microwave energy being transmitted or in one dimension the frequency of the microwave energy being detected. Some of these frequency scannable antennas are listed in U.S. Pat. No. 5,365,237 and that list is repeated here:
a) Waveguide antenna with slots PA1 b) Helical transmission line with taps PA1 c) Dielectric rod transmission line with taps PA1 d) Transmission grating PA1 e) Reflecting grating PA1 f) Dispersive prism PA1 g) Parallel plate transmission line with taps PA1 h) Dielectric slot transmission line with taps PA1 i) Stripline with taps PA1 j) Microstrip with taps PA1 k) Individual antennas interconnected with delavs lines
Another type of microwave antenna is the flat slot array antenna. Two such antennas are described in U.S. Pat. No. 5,173,714 issued Dec. 22, 1992 to Arimura et al and U.S. Pat. No. 5,177,496 issued on Jan. 5, 1993 to Arimura et al. These antennas appear to have been designed for use in microwave broadcasting of a microwave beam in a predetermined direction and contain no teaching regarding beam scanning. These prior art flat slot arrays have thicknesses of at least .lambda. (the antenna nominal wavelength) or greater and operate in a TE mode. Flat antennas with an effective thickness of .lambda. are herein referred to a resonant antenna.
What is needed is a better low cost microwave antenna that can be scanned in two dimensions and can be utilized for microwave imaging.